Project 3 will continue investigations of the cellular traffic in the bone marrow (BM). Using intravital microscopy[unreadable] (IVM), previous work in this project has shown how specialized endothelial cells (EC) in BM microvessels[unreadable] control the selective transit of blood cells into and out of the extravascular space.[unreadable] Aim 1 will characterize the nature and consequences of T cell interactions with dendritic cells (DCs) in the BM.[unreadable] This aim is based on the hypothesis that the BM is a gathering place for T cells and professional antigenpresenting[unreadable] cells that can foster the induction of antigen-specific cellular immune responses. We have shown[unreadable] recently that the BM is a reservoir and site of recruitment for CDS T cells, particularly central memory cells[unreadable] (Tcm). Preliminary data indicate that DCs can migrate from peripheral tissues to the BM, suggesting a potential[unreadable] mechanism for the induction of primary and secondary immune responses to peripheral antigens in the BM.[unreadable] However, it is not known how such immune responses are induced at the single-cell level and how immune[unreadable] responses in the BM differ from those in secondary lymphoid tissues. Subaim 1.1 will determine how different[unreadable] DC subsets are recruited to the BM and characterize the migratory behavior of homed DCs within the BM. This[unreadable] will include an analysis of plasmacytoid DCs (PDCs), which will be studied in a new transgenic mouse strain in[unreadable] which PDCs express GFP. In collaboration with Denisa Wagner, we will pursue preliminary observations that[unreadable] tissue-resident PDCs activate TNF-alpha converting enzyme (TACE). Subaim 1.2 will generate an in-depth[unreadable] analysis of naive T cell interactions with different DC subsets in the BM and subaim 1.3 will extend this[unreadable] analysis to explore DC-mediated recall responses by BM-resident Tcm. In collaboration with Leslie Silberstein[unreadable] we will investigate whether differential CXCR4 signaling is responsible for preliminary findings that different[unreadable] leukocyte subsets have distinct retention kinetics after homing to the BM. Subaim 1.4 will explore if T cell[unreadable] activation in BM induces cytotoxic effector cells (CTLs) and how CTLs function in this organ. Together, the[unreadable] experiments in aim 1 will clarify how blood cell transfusions containing T cells and/or DCs influence cellular[unreadable] immunity in the BM.[unreadable] Aim 2 will continue to study the mechanisms of multiple myeloma (MM) dissemination in the BM. This aim will[unreadable] also investigate the potential for CTL-mediated immunotherapy of MM, a highly aggressive plasma cell-derived[unreadable] tumor with pronounced BM tropism. The tumor cells are thought to arise during germinal center reactions in[unreadable] secondary lymphoid organs and must home via the blood to the BM to establish disease. Preliminary work for[unreadable] this aim has established an in vivo model to study MM migration and growth in murine BM. Subaim 2.1 will[unreadable] explore the molecular and cellular mechanisms of MM cell dissemination in the BM. Subaim 2.2 will pursue[unreadable] preliminary observations indicating that the CXCR4-CXCL12 pathway is critical for the survival and/or[unreadable] proliferation of MM cells in the BM. This work will be done in collaboration with Leslie Silberstein. Finally,[unreadable] subaim 2.3 will explore the feasibility of T cell-based anti-MM immunotherapy building on knowledge and[unreadable] techniques previously generated in this project. This aim will produce novel information that may improve[unreadable] clinical approaches in transfusion medicine and hematology/oncology to the treatment of MM.